The present invention relates generally to transmitters that measure fluid properties such as pressure in a process container. More specifically, the invention relates to a technique for mounting such transmitters or related process control equipment, e.g. a pressure diaphragm module known as a "remote seal" that connects to such a transmitter by a capillary line. The term "process instrument" as used herein includes all such transmitters and related process control equipment.
In FIG. 1, a PRIOR ART process instrument mount 10 is shown installed on a process container 12 such as a pipe, tank, or other (typically curved) vessel. The mount 10 is welded to the process container and is also threaded to receive a process instrument such as pressure transmitter 14. Once welded into position on the process container 12, the transmitter mount 10 becomes a permanent fixture to hold transmitter 14 in position on the container. Mount 10 has a base 10a and a flange 16 proximate such base. A gasket 18, sandwiched between flange 16 and a transmitter base 14a, is sized to mate with, or superimpose onto, the flange and seals the opening of the container to prevent leakage of process fluid from the container. Gasket 18 can be type Gylon No. 3510 available from Garlock Inc., Palmyra, N.Y. Transmitter 14 can be model 2090P available from Rosemount Inc., Eden Prairie, Minn.
Installation of mount 10 begins after evacuating process fluid from container 12. First, a hole is cut in the wall of container 12 at a selected installation position. Next, mount 10 is positioned in the hole such that base 10a is flush with an inner surface 12a (see FIG. 2) of container 12. A heat sink, screwed into mount 10 in place of transmitter 14, an be used by the installer to maneuver mount 10. A series of tack welds are then applied o the outer surface of mount 10 to temporarily hold the mount in position. If the temporary mount position is acceptable, a series of full welds are then applied. The full welds form a continuous weld seam or line (also known as a weld outlet interface line) along a circumference of the mount. After cooling, the heat sink is removed and the mount is ready to receive a gasket and process instrument.
FIG. 2 depicts a side view of a weld line 20 for a relatively small diameter process container 12. The plane of view of FIG. 2 is perpendicular to the plane of view of FIG. 1. Note that the weld line extends over an appreciable length L of mount 10. The smaller the diameter of container 12, the greater its curvature at the mount position, and the greater the extent L of the weld line along the mount.
A drawback of PRIOR ART mount 10 is its susceptibility to warpage during the welding operation, particularly for relatively small diameter containers 12. The intense heat produced during the process and the uneven distribution of loading along the length of the mount tends to distort the inner wall containing the threads. This distortion can prevent removal of the threaded heat sink used during welding, or can prevent a process instrument from being inserted into the mount. The mount 10 must then be removed and replaced, or rethreaded, causing delays and increased installation costs.
Therefore, there is a need for a transmitter mount that resists weld-induced warpage.